Ultrasonic imaging apparatuses used for medical imaging diagnoses are capable of observing, using an ultrasonic pulse echo method, a tomographic image of soft tissue of a living body, an image of blood flowing through a living body, or the like by displaying the image on a monitor substantially in real time. Further, nowadays, contrast-enhanced ultrasonography has been developed in which an ultrasonic contrast agent that contains stabilized microbubbles as the main component is administered through intravenous injection or the like, so that an image of the blood circulatory system or an organ such as a liver, which can easily be perfused with the contrast agent, is emphatically extracted.
Further, a microbubble has a characteristic that it functions as a nonlinear reflector, that is, it generates a readily detectable nonlinear echo even at a transmission ultrasound pressure in the range that ultrasonic diagnosis can be safely conducted. Such a characteristic can be utilized to discriminate an echo from the contrast agent from echoes from the surrounding body tissue. In the early days when the development of the contrast-enhanced ultrasonography using microbubbles was just started, a method was used in which the fundamental component included in a nonlinear echo signal was suppressed with the use of a bandpass filter to extract the second harmonic component. However, there has been a problem in that the use of the bandpass filter could increase the length of the echo signal in the time direction, and thereby degrading the axial resolution of the resulting ultrasonic echo image. Such a problem was solved with the pulse inversion method (Patent Document 1). Specifically, this method involves two times of transmission/reception with two transmission pulses whose polarities are reversed with respect to each other, and summing the obtained reception echo signals, so that the second harmonic component is extracted while the fundamental component is suppressed independently of a bandpass filter. Although this method would require twice the imaging time, it has a characteristic feature that it can extract nonlinear signal components without degradation of the axial resolution. Thus, a variety of methods have been developed based on such a method and have been put into practical use.
There has also been proposed a method for detecting inversion or non-inversion of a reception pulse in diagnostic ultrasonic imaging (Patent Document 2). In this method, the phase of the second harmonic echo component is detected through phase sensitive detection (PSD) in which the square of the fundamental echo component is used as a reference wave.    Patent Document 1: U.S. Pat. No. 6,095,980 B1    Patent Document 2: JP 2004-113364 A